Coated thin film disks are used in a variety of industries. One example is the computer hard disk industry. The computer hard disk (magnetic storage device) is a non-volatile memory device that can store large amounts of data.
An important specification in the design and manufacture of thin film media for hard disk drives is HMS_Wq, otherwise known as r.m.s. head-media spacing modulation. The HMS_Wq provides a measure of the spacing between a read/write head and the ultra-smooth surface of the thin film on the hard disk. An instrument currently used by designers to measure this parameter is known as the Candela profilometer. A description of such a profilometer is provided in U.S. Pat. No. 6,392,749.
The Candela profilometer infers the local slope of a disk surface by measuring the deflection of a collimated optical beam that is incident upon a disk surface at an oblique angle. One of the advantages of this technique is that it is affected very little by disk mode vibrations of the spinning disk under test. However, the technique is very sensitive to proper optical alignment and is not easily portable. This limits the usefulness of the Candela profilometer.
Another drawback to the arrangement provided by the Candela profilometer is that it normally measures both height and slope information. In order to obtain a measurement of just the height or the slope, special arrangements of the Candela profilometer are required, including employing multiple lasers to provide measurement signals.
Another type of method of measuring HMS_Wq is with a laser doppler vibrometer (LDV). Unlike the Candela profilometer, the LDV technique is a readily portable technique. However, because the LDV technique measures out-of-plane motion, the HMS_Wq measurement is easily contaminated by disk mode vibrations.